Ceramics are considered as the materials of future. However, the ceramics are notoriously brittle. The many potential applications of ceramics as high temperature structural materials have been hindered by their lack of toughness and reliability.
In order to improve the toughness and reliability of ceramics, some efforts have been directed towards the development of nanophase ceramics and nanophase ceramic composites. Dramatically different properties can be obtained by making ceramic composites and by making ceramic materials into nanograin structured materials. For examples, the toughness and strength of nanophase Al2O3—SiC composites are two to five times greater than those of the same materials with conventional structures, and a Si3N4—SiC nanophase composite can be stretched to two and a half times its original length at 1600° C.
Metallic carbide nanowhiskers, and in particular, silicon carbide (SiC) nanowhiskers (or nanofibrils) are of interest for their potential in the development of supertough ceramics matrix nanocomposites, and the preparation of metal matrix nanocomposites to improve the strength of metals. Potential applications of these nanowhiskers include nanophase carbides, nanophase ceramics, nanophase composites, and abrasive agents for high quality surface finish.
In order to achieve the desirable properties associated with SiC fibrils, or other metallic carbide fibrils, it is important that the fibrils have an extremely small, generally uniform diameter, substantially less than about 100 nm. Heretofore, however, it has not been possible to make SiC or other carbide fibrils in such extremely small dimensions.
SiC whiskers have been prepared by various methods, including hydrogen reduction of CH3SiCl3, vapor transport of Sic, catalytic reaction of SiO2 and carbon in rice hulls, and vapor-liquid-solid techniques. However, most of the SiC whiskers produced by these methods have poorly crystallized polycrystalline structure. Their diameters are often bigger than 0.5 micrometer.
NASA Tech Briefs LEW-15415/16 describes deposition of SiC onto carbon tow of 5-10 micrometers (basically conventional PAN based fibers) by chemical vapor deposition, principally by use of CH3SiCl3 and H2 as silicon deposition gas at a temperature of 1250 C. The resultant fibers had cracks and fissures, which were partially eliminated by incorporating the secondary pyrolytic carbon coating over the substrate. This document also includes a cursory account of an attempt to convert conventional carbon fibers (THORNEL M40) by using SiO gas at 1450 C for 30 minutes. An argon stream with 5-15% CO gas was used as a carrier gas to control the counter diffusion of SiO and CO gas. Moderate and widely varying strength (60-200 KSI) was achieved.
Zhou and Seraphin (Chemical Physics Letters 222 (May 13, 1994) 233-238, describe the preparation of single crystal SiC whiskers by direct reaction of carbon nanoclusters of DC arc generated nanotubes (i.e., clusters of buckytubes) retained on a carbon disc. The length and diameter of the carbon nanotubes ranged from 1-5 micrometer and 20-40 nm, respectively.
The disk containing the nanoclusters was placed on graphitic foil over SiO in a furnace and held at 1700 C, in a flowing argon atmosphere for two hours. The resulting SiC whiskers were about one order of magnitude longer and wider than the initial carbon nanotubes. The SiC whiskers would also continue to grow if the reaction was allowed to proceed.
Although the starting carbon nanotubes had diameters in the range of 20-40 nm, the smallest SiC whisker shown had a diameter greater than 100 nm. The lattice image of the whisker under TEM showed numerous defects.
It is believed that the resulting whiskers were frequently fused to one another at points of intersection. There are no distinguishing macrostructures or properties to relate the macroscopic morphology of the SiC fibers to the starting buckytube sample. Furthermore, the yield of SiC as fibers is low; only a small percentage of the carbon disc starting material (near the surface of the disc) was converted to SiC. Of the carbon material converted, a substantial amount is in the form of non-fibrous, amorphous particles or clumps.
Accordingly, although some attempts have been made to synthesize silicon carbide fibers and nanofibers, the prior efforts have not been successful in synthesizing high and consistent quality silicon carbide, or other carbide, nanofibrils predominantly in diameters substantially smaller than 100 nm.